The present invention relates to organic light emitting diode (OLED) structures, and more particularly, to top-emitting OLED structures comprising an emitting aperture.
Organic light emitting diode (OLED) devices are useful in a variety of applications such as watches, telephones, notebook computers, pagers, cell phones, calculators and the like. Conventional OLED structures are built on glass substrates in a manner such that a two-dimensional OLED array for image presentation is formed. Each OLED structure typically comprises an anode, a cathode and an organic emission layer interposed therebetween. When an electrical potential is present across the electrodes, holes and electrons are injected into the organic emission layer from the anode and the cathode, respectively. Light emission results from hole-electron recombination within the structure.
OLED structures are classified into two types, bottom-emitting and top-emitting. In an active matrix type OLED device, light generated from the bottom-emitting OLED structure, however, cannot penetrate the regions where TFTs (thin film transistors) and wirings are formed on a lower substrate, seriously reducing aperture ratio. Thus, top-emitting OLED structures are well suited for active matrix type OLED devices.
FIG. 1 is a sectional view of a conventional top-emitting OLED structure 100. A reflective electrode 120, serving as an anode, is formed on a substrate 110. The reflective electrode 120 can be an aluminum or silver layer. An organic emission layer 130 is formed on the reflective layer 120. A transparent metal layer 140, serving as a cathode, is formed on the organic emission layer 130. Note that the transparent metal layer 140 must be very thin (thinner than 50 Å) in order to be light transmissive. This, however, results in increased resistance of the metal layer 140 and decreases electron mobility and device performance. Additionally, the metal layer 140 absorbs some of the light output from the organic emission layer 130, thereby decreasing light emission efficiency. Moreover, the metal layer 140 reflects some of the emitted light, decreasing light emission efficiency due to the microcavity effect.
U.S. Pat. No. 6,670,772 to Arnold et al., the entirety of which is hereby incorporated by reference, describes a top-emitting OLED device comprising grating electrodes. The grating electrodes are employed to improve the light output of the disclosed OLED device.
U.S. Pat. No. 6,366,017 to Antoniadis et al., the entirety of which is hereby incorporated by reference, describes a top-emitting OLED device. In the disclosed OLED device, a distributed Bragg reflector is disposed on a cathode to increase brightness.
U.S. Pat. No. 5,115,442 to Lee et al., the entirety of which is hereby incorporated by reference, describes a top-emitting laser structure. In the laser structure, ion implantation is performed, creating an active region thus forming a resistance gradient therein. The resistance gradient causes an aperture in the laser structure.